According to Wikipedia.
Anal sex (also called anal intercourse) is the sex act in which the penis is inserted into the anus of a sexual partner. The term can also include other sexual acts involving the anus, including pegging, anilingus, fingering, and object insertion. While anal sex is commonly associated with male homosexuality, research shows that not all gay males engage in anal sex and that it is not uncommon in heterosexual relationships. Types of anal sex can also be a part of lesbian sexual practices. Many people find anal sex pleasurable, and some may reach orgasm—through stimulation of the prostate in men, and clitoral and G-Spot leg stimulation in women. However, many people find it painful as well, sometimes extremely so, which may be psychosomatic in some cases. As with most forms of sexual interaction, individuals are at risk for contracting sexually transmitted diseases, and therefore safe-sex practices are advised. Anal sex is considered a high-risk sexual practice, and unprotected anal sex is the riskiest of all forms of sexual intercourse, due to the vulnerability of the rectum and sphincter tissues. It generally requires a generous application of a personal lubricant to prevent tearing, since the anus does not have sufficient natural lubrication.
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Some men may enjoy being the insertive partner in anal sex because the anal sphincter is usually tighter than a vagina, which can yield greater tactile pleasure for the man via his penis. The attitude of women towards being the receptive partner in this practice is diverse: while some consider it painful or uncomfortable, others find it pleasurable and some even prefer it to vaginal intercourse. Unlike the clitoris or anus, the vaginal walls “contain relatively few nerve endings, making intense sexual stimulation, pleasure, and orgasm from vaginal-only penetration unlikely” and “it’s generally only the lower third of the vagina that has enough nerve endings to feel any stimulation at all from a penis, finger, toy, or other penetrative object”. Vaginal intercourse may, however, promote a satisfying feeling of fullness or closeness with a sexual partner. In a study of heterosexual anal sex – (8/2010 (n=214)) – female participants stated that stimulation to multiple erogenous zones simultaneously (the clitoris, the G-Spot, the anus, or other erogenous zones) enabled them to enjoy anal intercourse with much less discomfort compared to anal penetration by itself. When accompanied by one or more of the other stimuli, women were able to orgasm and those who did reported that an orgasm during anal sex is more of a full-body experience than an orgasm from clitoral stimulation alone. Anal sex is held to carry a very low risk of unwanted pregnancy when not accompanied with vaginal intercourse, as anal intercourse cannot lead to pregnancy unless sperm is somehow transported to the vaginal opening in the process; in some populations, this activity is frequently used as a means of contraception, often in the absence of a condom. The risk to the woman is greater than the risk to the man during male-to-female anal intercourse. The risk of injury to the receptive partner due to anal intercourse is many times higher than that due to vaginal sex, and the risk for transmission of HIV is higher for anal sex than for vaginal sex. Experts caution couples engaging in this practice to use protection, such as condoms, to stop the transmission of STDs. Additionally, the man should never move from anal sex immediately to vaginal sex while barebacking or without changing the condom, due to infections that can arise in the vagina by bacteria present within the anus; this also applies to the use of sex toys.
Historically, anal sex has been commonly associated with male homosexuality. However, many men who have sex with men do not engage in anal sex. Among men who have anal sex with other men, the insertive partner is called the top and the one being penetrated is called the bottom. Those who enjoy either role are referred to as versatile. Gay men who prefer anal sex may view it as “[their] version of intercourse” and as “the natural apex of sex, a wonderful expression of intimacy, and a great source of pleasure”. Psychologist Walt Odets said, “I think that anal sex has for gay men the same emotional significance that vaginal sex has for heterosexuals.” Some men who have sex with men prefer to engage in frot or other forms of mutual masturbation because they find it more pleasurable or more affectionate, to preserve technical virginity, or as safe sex alternatives to anal sex, while other frot advocates denounce anal sex as degrading to the receptive partner and unnecessarily risky.
Read more about anal sex at Wikipedia. Anal sex should ALWAYS be performed with anal lube.
